This invention involves the reading of magnetically recorded data and, specifically, deals with new designs for magnetic sensors which can act as magnetic read heads utilizing the giant magneto-resistance effect.
Devices utilizing the giant magneto-resistance (GMR) effect have utility as magnetic sensors, especially as read sensors in read heads used in magnetic disc storage systems. The GMR effect is observed in thin, electrically conductive multi-layer systems having multiple magnetic layers. One sensor type that utilizes the GMR effect is the GMR multilayer. The GMR multilayer typically comprise a series of bi-layer devices, each of which comprise a thin sheet of a ferromagnetic material and a thin sheet of a non-magnetic material. The bi-layers are stacked to form a multi-layer device. The multi-layer device is usually mounted in the read head so that the magnetic axis of the ferromagnetic layers are transverse to the direction of rotation of the disc and parallel to the plane of the disc.
In operation, a sense current is caused to flow through the read head and therefore through the sensor. The magnetic flux from the disc causes a rotation of the magnetization vector in at least one of the sheets, which in turn causes a change in the overall resistance of the sensor. As the resistance of the sensor changes, the voltage across the sensor changes, thereby producing an output voltage.
The output voltage produced by the sensor is affected by various characteristics of the sensor. The sense current can flow through the sensor in a direction that is parallel to the planes of the layers or stacked strips. This is known as a current-in-plane or CIP configuration. Alternatively, the sense current can flow through the sensor in a direction that is perpendicular to the planes of the layers or stacked strips that comprise the sensor. This configuration is known as a current-perpendicular-to-plane component or CPP.
The types of sensors commonly in use today for magnetic read heads for a magnetic disc can be categorized as Current-In-Plane (CIP) sensors. Such a sensor is shown schematically in FIG. 1. In FIG. 1, the current is represented by arrow 8 and is shown flowing parallel to layers 9 of the sensor.
CPP sensors are known but are not commonly used for the reading of data from a magnetic disc. A CPP sensor is shown schematically in FIG. 2, wherein the current is represented by arrow 8 and is shown flowing perpendicular to layers 9.
The CPP sensor is interesting because of its potentially larger giant magneto-resistance (GMR) or change in resistance when a magnetic field is applied. The CPP sensor is therefore capable of producing a higher output voltage than the CIP sensor, which results in a more precise and sensitive read head. The larger change in resistance comes about because all of the current needs to pass through every ferromagnetic/non-magnetic/ferromagnetic (FM/NM/FM) series of interfaces and none of the current is shunted around the interfaces. Because every film and interface leads to additional resistance, it is desirable to have all of the layers and interfaces contribute to the overall xcex94 R of the device.
One example of a CPP sensor is a GMR multi-layer, which consists of a series of FM/NM bi-layers, as shown in FIG. 2. Every series of interfaces is an opportunity for interfacial spin-dependent scattering and every FM material is a opportunity for bulk spin-dependent bulk.
An example of a transfer curve from a CPP GMR multi-layer made of 20 bi-layers of (Cu 18 xc3x85 CoFe 10 xc3x85) is shown FIG. 3. In the quiescent state, the magnetization of adjacent layers in this sample are oriented 180xc2x0 with respect to each other, due to RKKY coupling, as is well known in the art. The thickness of the Cu layers was chosen such that the coupling between the CoFe layers would be anti-ferromagnetic (AFM).
It can be seen from FIG. 3 that if this type of sensor is to be used in a magnetic read head, it will be necessary to bias the sensor such that it operates in a linear region, such as the regions denoted by reference numbers 1 and 2 in FIG. 3. This will be necessary if detection schemes that depend on linearity are to be used.
It is critical that the GMR multi-layer sensor be biased properly in its linear region of operation. If the sensor is either over or under biased, the signal will become non-linear and create a loss of amplitude, signal asymmetry, detection problems and tracking problems.
One way of biasing a GMR multi-layer sensor is to place a permanent magnet (PM) nearby, such that the magnetizations of adjacent FM layers are oriented 90xc2x0 with respect to each other. This would be similar to applying a DC magnetic field of xcx9c500 Oe to the sensor whose transfer curve is shown in shown in FIG. 3. The sensor could then be used to sense the field from a magnetic recording media.
One possible CPP read head design using a permanent magnet to bias the sensor into the linear operating region is shown schematically in FIG. 4. This design uses a permanent magnet and uses the shields as the current carrying leads.
One problem with this configuration is that the positioning of the permanent magnet to properly bias the GMR layer is not a trivial task. It may be critical to control the spacing between the GMR multi-layer and the permanent magnet and to center the GMR multi-layer between the ends of the permanent magnet.
When building the read head, the actual bias point cannot be determined exactly until after the read head is almost completely built. If the bias point is not correct and there is no way of adjusting the bias, the head will need to be scrapped and all of the previous processing will have been wasted.
To avoid expensive manufacturing costs associated with the positioning of the permanent magnet and to avoid scrapping a high percentage of the sensors due to inability to properly bias them, it would be desirable to have a way to adjust the biasing on the sensor such that it operates in a linear region of the transfer curve.
This invention describes an effective way of biasing the GMR multi-layer using the sense current. This biasing may be able to be used in lieu of a permanent magnet, such that the bias is adjustable, which would greatly simplify the sensor manufacturing process, or it may be used in conjunction with a permanent magnet to xe2x80x9cfine tunexe2x80x9d the biasing provided by the permanent magnet.
In this invention, the sense current will be used to induce spin-momentum transfer from one FM layer to the next FM layer to cause the magnetization of these layers to align 90xc2x0 with respect to each other, thereby providing the proper biasing of the sensor.